In the field of artificial heart-assist technology, advances in the mechanical design of implantable circulatory pumps have been complemented by similar improvements in their external control units. These units have been designed to be increasingly sensitive to the dynamic interaction between the blood pump and the patient's heart to better provide end-users with real-time information that can be used to measure the machine's performance under particular operating parameters.
While blood flow measurement through the artificial heart pump is most accurate when received directly from an actual probe inserted into the outflow track of the pump conduit, implanted sensors that maintain direct contact with blood can perturb its flow and activate factors within the blood that increase the risk of blood clot formation. These clots can disturb motor blade performance and cause complete machine malfunction. When this occurs, total pump replacement may be required, mandating a high-risk open chest surgery. In the United States, direct flow heart pumps are limited, by regulation, to investigational use only. Only indirect flow measurement technology is approved by the Food and Drug Administration (FDA) for widespread therapeutic use in the United States, and heart pump engineering trends are producing new indirect-flow prototypes that are more reliable and durable than previous iterations because they continue to reduce the number of high resistance areas in the interface between the pump's surface and the patient's blood. Currently, the newest “3rd generation” heart pump models all estimate flow indirectly without the use of direct sensor components.
As an alternative to the placement of direct measurement sensors in the body, heart pump performance can be indirectly estimated using engineering formulas that relate motor torque, fluid flow rate, and the pressure of fluid moved by the motor at a particular speed. For most heart pumps, the speed of the moving motor blades that propel blood forward through the heart is the one fixed variable that can be adjusted by the end-user. Because the motors are based on brushless, direct-current design, the torque they generate is directly proportional to the power they consume in electrical current. By measuring the real-time power demands of a pump, the torque variable can be estimated. Once both torque and speed variables are known, flow and pump head pressure (the difference in pressure between the inflow and outflow portions of the pump) can be estimated. Heart pumps differ from each other in certain design characteristics that cause them to react idiosyncratically when they consume power to propel blood through the circulation. As a result, each pump utilizes its own proprietary program to convert current usage to torque and to calculate the effects of blood viscosity on the motor.
In its current design configuration, to obtain real-time flow measurements for heart pump technology based on indirect flow estimations, a portable controller must be attached to a larger monitor. Depending on the manufacturer, the monitor contains the software to display continuous flow data, either numerically alone or both numerically and in a waveform format. These monitors are cost prohibitive for individual patient use, and only intended for diagnosing and managing heart pump-related issues in hospital or clinic facilities where expert personnel are available to titrate the motor speed to optimal flow dynamics. After hospital discharge, and between outpatient clinic visits every several months, the heart pump device functions according to the most recent settings and cannot respond dynamically to any change in the patient's physiology.
The inability to remotely discern heart pump operating variables, characterize their trends over time, and assess their interactions with the native heart represents a missed opportunity to better utilize this transformative technology. The infrequent contact between expert and patient due to logistical constraints results in a necessarily passive heart pump utilization strategy that precludes maximal titration of the heart pump to respond to the patient's changing circulatory status. High motor rotating speeds can cause greater emptying of the left heart than physiologically appropriate and precipitate suction problems between the inlet port and the heart wall, especially in low volume situations. Because patients receive limited supervision as outpatients, clinicians and engineers may enter long-term speed settings that are lower than optimal to provide a safety margin against heart pump suction complications. However, this added safety is gained at the cost of the pump's full potential to support the heart, which could be achieved with higher operating speeds. This is undesirable because heart failure is a dynamic process: either the patient's native circulatory function is improving with the support of the pump and gradually recovering lost function or it is further deteriorating as a result of intrinsic, irreversible disease—opposite trends that require divergent heart pump management strategies. Level of patient daily activity and hydration status are other variable factors that can influence the optimal operating target speed of a particular heart pump.
A device with the capability to remotely monitor disease progression in patients with implanted artificial heart pumps, including those using the indirect-flow measurement algorithms on which the great majority of heart pumps operate, could form the cornerstone of a more nuanced heart pump management strategy by facilitating expert oversight of the patient and the implanted artificial heart pump in a non-clinical setting. A system employing such a device could utilize software packages to create dynamic models of the heart pump flow patterns and the patient's cardiac performance to guide fine-tuning of speed adjustments. Such an innovation would be useful for ensuring the most satisfactory pump performance and identifying problem trends requiring early intervention to prevent severe complications, including hospitalization or death.